Derivative Instruments (Forward Currency Exchange Contracts) (Details) - Foreign Exchange Contract - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount	$ 5,330	$ 5,871
Maximum Term (Days)	586 days	548 days
Derivative, Fair Value, Net	$ 63	$ (21)
Other current assets
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	74	15
Other noncurrent assets
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	2	1
Other current liabilities
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(12)	(36)
Other noncurrent liabilities
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(1)	(1)
Designated
Derivative [Line Items]
Notional Amount	782	1,104
Derivative, Fair Value, Net	5	4
Designated | Other current assets
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	12	11
Designated | Other noncurrent assets
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	2	1
Designated | Other current liabilities
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(8)	(7)
Designated | Other noncurrent liabilities
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(1)	(1)
Non-Designated
Derivative [Line Items]
Notional Amount	4,548	4,767
Derivative, Fair Value, Net	58	(25)
Non-Designated | Other current assets
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	62	4
Non-Designated | Other noncurrent assets
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0
Non-Designated | Other current liabilities
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(4)	(29)
Non-Designated | Other noncurrent liabilities
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ 0	$ 0